1. Field of the Invention
The present invention relates to an image reading apparatus using an image sensor such as a CCD (charge coupled device), and more particularly to an image reading apparatus which includes an original table having noticeable variations in transmittancy.
2. Description of the Related Art
Hitherto, in an attempt to compensate for variations in the outputs of image sensors such as CCDs, an image reading apparatus such as a copier or a facsimile machine performs shading correction in which the image sensors are used to read a reference white board having a constant density, and the lamp voltage, circuit gain, and the like are controlled so that the read data reaches a predetermined value.
In an image reading apparatus having a mechanism for automatically feeding an original onto a glass original table, the glass original table may be sometimes implemented as an EC-coated glass plate, i.e., one which is coated with an electrically conductive coating agent in order to prevent or reduce static electricity which is produced by friction between the glass original table and the original. In this case, a reference white board for shading correction is also read by an image sensor through a normal glass plate that is not coated with such an electrically conductive coating agent.
Since an EC-coated glass plate has a slightly lower transmittancy than a normal glass plate, the R, G, and B signal levels are each lowered, by approximately 4 to 6%, as compared to when the same original is read through a normal glass plate. Therefore, one problem is that the signal levels cannot be precisely corrected for even if an original image signal which is read through the EC-coated glass plate is subjected to shading correction based on the reference white board data read through the normal glass plate.
In order to overcome this problem, as disclosed in Japanese Patent Laid-Open No. 7-273954, a contemplated method is to perform shading correction on an original image signal taking into account a difference in transmittancy between a normal glass plate which carries a reference white board and a glass original table which is coated with an electrically conductive coating agent.
In this approach, however, the transmittancies of the normal glass plate and EC-coated glass plate are uniquely defined, and variations in transmittancy from one EC-coated glass plate to another are not considered at all.
In production, the transmittancy of EC-coated glass plates varies over a range of about ±2%, leading to the problem that image reading level is different from the inherent signal level. With an EC-coated glass plate having a higher transmittancy, obscure detail in a highlighted area is revealed; with one having a lower transmittancy, conversely, the image becomes darker, with a foggy background.
In a typical image reading apparatus, light emitted from a light source passes through a glass original table is reflected by an original, again passes through the glass original table, and is then focused on a CCD through a mirror and a lens. Since light passes through the glass original table twice, a difference in transmittancy due to variations in transmittancy of EC-coated glass plates may doubly affect the reading level.
Furthermore, a problem involved with an apparatus which uses a scanner to read printer output patches and which corrects for a printer gradation characteristic is that variations in transmittancy of EC-coated glass plates cause the read signals of gradation patches to vary in the printer outputs, thereby preventing a precise gradation correction. This problem is noticeable, in particular, with an EC-coated glass plate having a higher transmittancy, such that reading of gradation patches in a highlighted area is saturated, causing a pseudo-outline.